In […], John Harrison invented the marine chronometer to establish the longitude of a ship at sea::1737 In 1737, […] invented the marine chronometer to establish the longitude of a ship at sea::John Harrison In 1737, John Harrison invented the […] to establish the longitude of a ship at sea::marine chronometer In 1737, John Harrison invented the marine chronometer to […]::establish the longitude of a ship at sea […] was the first to use the sun clock, doing so in 3500 BC. It used buildings known as obelisks to accomplish this.::Ancient Egypt Ancient Egypt was the first to use the […], doing so in 3500 BC. It used buildings known as obelisks to accomplish this.::sun clock Ancient Egypt was the first to use the sun clock, doing so in […]. It used buildings known as obelisks to accomplish this.::3500 BC Ancient Egypt was the first to use the sun clock, doing so in 3500 BC. It used buildings known as […] to accomplish this.::obelisks The […], invented in 600 BCE, made it possible to measure night-time hours (function). When being operated, two were used to establish a north-south meridian with the Pole star.::Merkhet The Merkhet, invented in […], made it possible to measure night-time hours (function). When being operated, two were used to establish a north-south meridian with the Pole star.::600 BCE The Merkhet, invented in 600 BCE, […] (function). When being operated, two were used to establish a north-south meridian with the Pole star.::made it possible to measure night-time hours The Merkhet, invented in 600 BCE, made it possible to measure night-time hours (function). When being operated, two were used to establish a […] with the Pole star.::north-south meridian The Merkhet, invented in 600 BCE, made it possible to measure night-time hours (function). When being operated, two were used to establish a north-south meridian with the […].::Pole star The […] was first used by Greeks around 325 BCE. The oldest was found in the tomb of Egyptian pharaoh Amenhotep I, around 1500 BCE.::clepsydra The clepsydra was first used by […] around 325 BCE. The oldest was found in the tomb of Egyptian pharaoh Amenhotep I, around 1500 BCE.::Greeks The clepsydra was first used by Greeks around […]. The oldest was found in the tomb of Egyptian pharaoh Amenhotep I, around 1500 BCE.::325 BCE The clepsydra was first used by Greeks around 325 BCE. The oldest was found in the tomb of […], around 1500 BCE.::Egyptian pharaoh Amenhotep I The clepsydra was first used by Greeks around 325 BCE. The oldest was found in the tomb of Egyptian pharaoh Amenhotep I, around […].::1500 BCE Two types of […] exist, inflow and outflow::water clocks Two types of water clocks exist, […]::inflow and outflow The […] used the clepsydra to drive various mechanisms, one being to illustrate astronomical phenomena.::Chinese The Chinese used […] to drive various mechanisms, one being to illustrate astronomical phenomena.::the clepsydra The Chinese used the clepsydra to drive various mechanisms, one being to illustrate […].::astronomical phenomena […], in the 11th century AD (or 1090), built an elaborate clock tower, whose height was over 30 feet. Inside the tower was/were manikins which rang bells or gongs and tablets that indicated the hour.::Su Sung, also called Su Song or Zirong Su Sung, also called Su Song or Zirong, in […], built an elaborate clock tower, whose height was over 30 feet. Inside the tower was/were manikins which rang bells or gongs and tablets that indicated the hour.::the 11th century AD (or 1090) Su Sung, also called Su Song or Zirong, in the 11th century AD (or 1090), built […], whose height was over 30 feet. Inside the tower was/were manikins which rang bells or gongs and tablets that indicated the hour.::an elaborate clock tower Su Sung, also called Su Song or Zirong, in the 11th century AD (or 1090), built an elaborate clock tower, whose height was […]. Inside the tower was/were manikins which rang bells or gongs and tablets that indicated the hour.::over 30 feet Su Sung, also called Su Song or Zirong, in the 11th century AD (or 1090), built an elaborate clock tower, whose height was over 30 feet. Inside the tower was/were […].::manikins which rang bells or gongs and tablets that indicated the hour In […], Andronikos of Cyrrhus, a Macedonian astronomer, constructed his Horologion (today known as the Tower of the Winds), in the Athens marketplace.::the first half of the 1st century BC In the first half of the 1st century BC, […], a Macedonian astronomer, constructed his Horologion (today known as the Tower of the Winds), in the Athens marketplace.::Andronikos of Cyrrhus In the first half of the 1st century BC, Andronikos of Cyrrhus, a […], constructed his Horologion (today known as the Tower of the Winds), in the Athens marketplace.::Macedonian astronomer In the first half of the 1st century BC, Andronikos of Cyrrhus, a Macedonian astronomer, constructed his […], in the Athens marketplace.::Horologion (today known as the Tower of the Winds) In the first half of the 1st century BC, Andronikos of Cyrrhus, a Macedonian astronomer, constructed his Horologion (today known as the Tower of the Winds), in the […].::Athens marketplace The […] featured a 24 hour mechanized clepsydra and also had indicators for the eight winds in Greek myths, for which the tower got its name. In terms of timetelling abilities, it could display the seasons of the year and astrological dates and periods.::Tower of the Winds or Horologion The Tower of the Winds or Horologion featured a […] and also had indicators for the eight winds in Greek myths, for which the tower got its name. In terms of timetelling abilities, it could display the seasons of the year and astrological dates and periods.::24 hour mechanized clepsydra The Tower of the Winds or Horologion featured a 24 hour mechanized clepsydra and also had […], for which the tower got its name. In terms of timetelling abilities, it could display the seasons of the year and astrological dates and periods.::indicators for the eight winds in Greek myths The Tower of the Winds or Horologion featured a 24 hour mechanized clepsydra and also had indicators for the eight winds in Greek myths, for which the tower got its name. In terms of timetelling abilities, it could display […].::the seasons of the year and astrological dates and periods The […] was the favored device to tell time in the Middle Ages.::sundial The sundial was the favored device to tell time in […].::the Middle Ages One version of a […] was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer). For time, it was scribed with hour lines for different seasons.::sundial One version of a sundial was the […], a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer). For time, it was scribed with hour lines for different seasons.::hemispherical dial One version of a sundial was the hemispherical dial, […], carrying a central vertical gnomon (pointer). For time, it was scribed with hour lines for different seasons.::a bowl-shaped depression cut into a block of stone One version of a sundial was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical […]. For time, it was scribed with hour lines for different seasons.::gnomon (pointer) One version of a sundial was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer). For time, it was scribed with […].::hour lines for different seasons By […], Vitruvius could describe 13 different sundial styles used in Greece, Asia Minor, and Italy.::30 BC By 30 BC, […] could describe 13 different sundial styles used in Greece, Asia Minor, and Italy.::Vitruvius By 30 BC, Vitruvius could describe […] used in Greece, Asia Minor, and Italy.::13 different sundial styles By 30 BC, Vitruvius could describe 13 different sundial styles used in […].::Greece, Asia Minor, and Italy In […], the Prague Astronomical Clock is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had a circle with zodiac signs. To represent the vernal equinox, there is a small star, and it could also read in sidereal time.::the 1400s In the 1400s, the […] is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had a circle with zodiac signs. To represent the vernal equinox, there is a small star, and it could also read in sidereal time.::Prague Astronomical Clock In the 1400s, the Prague Astronomical Clock is built. For timetelling it is […]. It also had a circle with zodiac signs. To represent the vernal equinox, there is a small star, and it could also read in sidereal time.::an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset In the 1400s, the Prague Astronomical Clock is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had […]. To represent the vernal equinox, there is a small star, and it could also read in sidereal time.::a circle with zodiac signs In the 1400s, the Prague Astronomical Clock is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had a circle with zodiac signs. To represent the vernal equinox, there is a small star, and it could also read in […].::sidereal time In the 1400s, the Prague Astronomical Clock is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had a circle with zodiac signs. To represent […], there is a small star, and it could also read in sidereal time.::the vernal equinox In the 1400s, the Prague Astronomical Clock is built. For timetelling it is an hourly clock with curved lines that represent 1/12 of the time between sunrise and sunset. It also had a circle with zodiac signs. To represent the vernal equinox, there is a […], and it could also read in sidereal time.::small star The […] was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by Richard of Wallingford.::verge escapement (or crown wheel escapement) The verge escapement (or crown wheel escapement) was used in […]. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by Richard of Wallingford.::the 14th through mid 19th centuries The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of […] possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by Richard of Wallingford.::all-mechanical clocks The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from […]. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by Richard of Wallingford.::continuous processes to repetitive, oscillatory processes The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the […], a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by Richard of Wallingford.::foliot The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in […] in the Tractatus Horologii Astronimici, by Richard of Wallingford.::1327 The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the […], by Richard of Wallingford.::Tractatus Horologii Astronimici The verge escapement (or crown wheel escapement) was used in the 14th through mid 19th centuries. It made the development of all-mechanical clocks possible, shifting the measuring of time from continuous processes to repetitive, oscillatory processes. It was commonly used with the foliot, a primitive balance wheel. A variant of it was first described in 1327 in the Tractatus Horologii Astronimici, by […].::Richard of Wallingford The driving force to a […] is gravity.::verge escapement The driving force to a verge escapement is […].::gravity In […], pocket watches were invented in Tudor England. Weirdly enough, to begin with they were worn around the neck and were not very accurate timekeepers, instead being more decorative.::the 1500s In the 1500s, […] were invented in Tudor England. Weirdly enough, to begin with they were worn around the neck and were not very accurate timekeepers, instead being more decorative.::pocket watches In the 1500s, pocket watches were invented in […]. Weirdly enough, to begin with they were worn around the neck and were not very accurate timekeepers, instead being more decorative.::Tudor England In […], spring driven clocks were accurate to within a minute a day. They were invented by Peter Heinlein of Nuremberg.::1510 In 1510, […] were accurate to within a minute a day. They were invented by Peter Heinlein of Nuremberg.::spring driven clocks In 1510, spring driven clocks were accurate to within […]. They were invented by Peter Heinlein of Nuremberg.::a minute a day In 1510, spring driven clocks were accurate to within a minute a day. They were invented by […].::Peter Heinlein of Nuremberg In […], Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::1656 In 1656, […] made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::Christiaan Huygens In 1656, Christiaan Huygens made the first […], after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::pendulum In 1656, Christiaan Huygens made the first pendulum, after […] became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::Galileo In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in […]. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::1582 In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was […]. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::less than 1 minute per day In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In […], Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::1657 In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, […] created the balance wheel to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::Huygens In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the […] to work alongside it and to reduce the clock's error even farther, to less than 10 seconds per day.::balance wheel In 1656, Christiaan Huygens made the first pendulum, after Galileo became the first to describe it in 1582. The error of the first pendulum was less than 1 minute per day. In 1657, Huygens created the balance wheel to work alongside it and to reduce the clock's error even farther, to […].::less than 10 seconds per day In the 1500s, pocket watches were invented in Tudor England. Weirdly enough, to begin with they were […] and were not very accurate timekeepers, instead being more decorative.::worn around the neck In the 1500s, pocket watches were invented in Tudor England. Weirdly enough, to begin with they were worn around the neck and were […].::not very accurate timekeepers, instead being more decorative In […], grandfather clocks were built::the mid 1660s In the mid 1660s, […] were built::grandfather clocks In […], William Clement began building clocks with the new anchor escapement (also known as recoil escapement). It was an improvement over the verge escapement due to its lower level of interference with the motion of pendulums.::1671 In 1671, […] began building clocks with the new anchor escapement (also known as recoil escapement). It was an improvement over the verge escapement due to its lower level of interference with the motion of pendulums.::William Clement In 1671, William Clement began building clocks with the […]. It was an improvement over the verge escapement due to its lower level of interference with the motion of pendulums.::new anchor escapement (also known as recoil escapement) In 1671, William Clement began building clocks with the new anchor escapement (also known as recoil escapement). It was an improvement over the […] due to its lower level of interference with the motion of pendulums.::verge escapement In 1671, William Clement began building clocks with the new anchor escapement (also known as recoil escapement). It was an improvement over the verge escapement due to its lower level of interference with the […].::motion of pendulums In […], John Arnold invented jeweling. In this process, precious stones such as rubies were used as bearings to reduce friction.::the time period near the 1760s In the time period near the 1760s, […] invented jeweling. In this process, precious stones such as rubies were used as bearings to reduce friction.::John Arnold In the time period near the 1760s, John Arnold invented […]. In this process, precious stones such as rubies were used as bearings to reduce friction.::jeweling In the time period near the 1760s, John Arnold invented jeweling. In this process, […].::precious stones such as rubies were used as bearings to reduce friction In […], pocket chronometers became available. They added a second hand as well.::1800 In 1800, […] became available. They added a second hand as well.::pocket chronometers In 1800, pocket chronometers became available. They added […] as well.::a second hand In […], Pierre Curie discovered that the application of pressure to a quartz crystal causes it to vibrate at a constant frequency.::1880 In 1880, […] discovered that the application of pressure to a quartz crystal causes it to vibrate at a constant frequency.::Pierre Curie In 1880, Pierre Curie discovered that the […].::application of pressure to a quartz crystal causes it to vibrate at a constant frequency In […], WA Marrison built the first quartz clock, which could measure the accuracy of a clock to a millionth of a second.::1927 In 1927, […] built the first quartz clock, which could measure the accuracy of a clock to a millionth of a second.::WA Marrison In 1927, WA Marrison built the first quartz clock, which could measure the accuracy of a clock to […].::a millionth of a second In […], refinements to the clocks of Siegmund Riefler with a nearly free pendulum became accurate to a hundredth of a second a day. As a result, it became the standard in many astronmical observatories.::1889 In 1889, refinements to the clocks of […] with a nearly free pendulum became accurate to a hundredth of a second a day. As a result, it became the standard in many astronmical observatories.::Siegmund Riefler In 1889, refinements to the clocks of Siegmund Riefler with a […] became accurate to a hundredth of a second a day. As a result, it became the standard in many astronmical observatories.::nearly free pendulum In 1889, refinements to the clocks of Siegmund Riefler with a nearly free pendulum became accurate to […]. As a result, it became the standard in many astronmical observatories.::a hundredth of a second a day In 1889, refinements to the clocks of Siegmund Riefler with a nearly free pendulum became accurate to a hundredth of a second a day. As a result, it […].::became the standard in many astronmical observatories In […], RJ Rudd introduced the true free-pendulum principle, which stimulated development on several free-pendulum clocks.::1898 In 1898, […] introduced the true free-pendulum principle, which stimulated development on several free-pendulum clocks.::RJ Rudd In 1898, RJ Rudd introduced the […], which stimulated development on several free-pendulum clocks.::true free-pendulum principle In 1898, RJ Rudd introduced the true free-pendulum principle, which stimulated development on several […].::free-pendulum clocks In […], the WH Shortt clock was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first gentle pushes to maintain its motion and also drove the clocks' hands. The first would then remain free from mechanical tasks that would disturb its regularity.::1921 In 1921, the […] was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first gentle pushes to maintain its motion and also drove the clocks' hands. The first would then remain free from mechanical tasks that would disturb its regularity.::WH Shortt clock In 1921, the WH Shortt clock was demonstrated. It almost immediately replaced […] in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first gentle pushes to maintain its motion and also drove the clocks' hands. The first would then remain free from mechanical tasks that would disturb its regularity.::Riefler's clock In 1921, the WH Shortt clock was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, […]. The second gave the first gentle pushes to maintain its motion and also drove the clocks' hands. The first would then remain free from mechanical tasks that would disturb its regularity.::a master and a slave pendulum In 1921, the WH Shortt clock was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first […] and also drove the clocks' hands. The first would then remain free from mechanical tasks that would disturb its regularity.::gentle pushes to maintain its motion In 1921, the WH Shortt clock was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first gentle pushes to maintain its motion and also […]. The first would then remain free from mechanical tasks that would disturb its regularity.::drove the clocks' hands In 1921, the WH Shortt clock was demonstrated. It almost immediately replaced Riefler's clock in observatories. The clock had two pendulums, a master and a slave pendulum. The second gave the first gentle pushes to maintain its motion and also drove the clocks' hands. The first would then […].::remain free from mechanical tasks that would disturb its regularity In […], the development of radar and extremely high frequency radio communication made the generation of electromagnetic waves needed to interact with atoms possible. This research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule.::the 1930s and 1940s In the 1930s and 1940s, the development of […] made the generation of electromagnetic waves needed to interact with atoms possible. This research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule.::radar and extremely high frequency radio communication In the 1930s and 1940s, the development of radar and extremely high frequency radio communication made the […] possible. This research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule.::generation of electromagnetic waves needed to interact with atoms In the 1930s and 1940s, the development of radar and extremely high frequency radio communication made the generation of electromagnetic waves needed to interact with atoms possible. This research aimed at developing an […] focused first on microwave resonances in the ammonia molecule.::atomic clock In the 1930s and 1940s, the development of radar and extremely high frequency radio communication made the generation of electromagnetic waves needed to interact with atoms possible. This research aimed at developing an atomic clock focused first on […] in the ammonia molecule.::microwave resonances In the 1930s and 1940s, the development of radar and extremely high frequency radio communication made the generation of electromagnetic waves needed to interact with atoms possible. This research aimed at developing an atomic clock focused first on microwave resonances in the […].::ammonia molecule In […], the National Institute for Standards and Technology created the first atomic clock, based on ammonia. However, it was not much better than existing standards, causing attention to focus more on atomic-beam devices based on Cesium.::1949 In 1949, […] created the first atomic clock, based on ammonia. However, it was not much better than existing standards, causing attention to focus more on atomic-beam devices based on Cesium.::the National Institute for Standards and Technology In 1949, the National Institute for Standards and Technology created […], based on ammonia. However, it was not much better than existing standards, causing attention to focus more on atomic-beam devices based on Cesium.::the first atomic clock In 1949, the National Institute for Standards and Technology created the first atomic clock, based on […]. However, it was not much better than existing standards, causing attention to focus more on atomic-beam devices based on Cesium.::ammonia In 1949, the National Institute for Standards and Technology created the first atomic clock, based on ammonia. However, it was not much better than existing standards, causing attention to focus more on […] based on Cesium.::atomic-beam devices In 1949, the National Institute for Standards and Technology created the first atomic clock, based on ammonia. However, it was not much better than existing standards, causing attention to focus more on atomic-beam devices based on […].::Cesium In […], wristwatches were made with more robust mechanisms to make them waterproof, shockproof, and able to function under extreme pressure. Before this, wristwatches made by machine were given to soldiers during World War I.::1945 In 1945, […] were made with more robust mechanisms to make them waterproof, shockproof, and able to function under extreme pressure. Before this, wristwatches made by machine were given to soldiers during World War I.::wristwatches In 1945, wristwatches were made with more robust mechanisms to make them […]. Before this, wristwatches made by machine were given to soldiers during World War I.::waterproof, shockproof, and able to function under extreme pressure In 1945, wristwatches were made with more robust mechanisms to make them waterproof, shockproof, and able to function under extreme pressure. Before this, wristwatches made by machine were given to […] during World War I.::soldiers In 1945, wristwatches were made with more robust mechanisms to make them waterproof, shockproof, and able to function under extreme pressure. Before this, wristwatches made by machine were given to soldiers during […].::World War I In […], battery-power watches began to be embraced, particularly by Asian watch manufactures, especially those in Japan.::1952 In 1952, […] began to be embraced, particularly by Asian watch manufactures, especially those in Japan.::battery-power watches In 1952, battery-power watches began to be embraced, particularly by […].::Asian watch manufactures, especially those in Japan In […], L Essen and J Perry, two English physicists, constructed the first practical Cesium atomic frequency standard at the national Physical Laboratory in England in collaboration with the US Naval Observatory and measured relatively with astronomical time.::1955 In 1955, […] constructed the first practical Cesium atomic frequency standard at the national Physical Laboratory in England in collaboration with the US Naval Observatory and measured relatively with astronomical time.::L Essen and J Perry, two English physicists, In 1955, L Essen and J Perry, two English physicists, constructed the […] at the national Physical Laboratory in England in collaboration with the US Naval Observatory and measured relatively with astronomical time.::first practical Cesium atomic frequency standard In 1955, L Essen and J Perry, two English physicists, constructed the first practical Cesium atomic frequency standard at […] in collaboration with the US Naval Observatory and measured relatively with astronomical time.::the national Physical Laboratory in England In 1955, L Essen and J Perry, two English physicists, constructed the first practical Cesium atomic frequency standard at the national Physical Laboratory in England in collaboration with […] and measured relatively with astronomical time.::the US Naval Observatory In 1955, L Essen and J Perry, two English physicists, constructed the first practical Cesium atomic frequency standard at the national Physical Laboratory in England in collaboration with the US Naval Observatory and measured relatively with […].::astronomical time In […], Cesium standards began to be used by NIST, along with other institutions, leading to widspread usage of the new technology.::1960 In 1960, […] began to be used by NIST, along with other institutions, leading to widspread usage of the new technology.::Cesium standards In 1960, Cesium standards began to be used by […], along with other institutions, leading to widspread usage of the new technology.::NIST In […], the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::1967 In 1967, […] was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::the cesium atom In 1967, the cesium atom was formally recognized internationally as the time standard. In the […], it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::SI unit definition of a second In 1967, the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of […], as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::Earth motions In 1967, the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of […], the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::2002 In 1967, the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to […]. The latest clock built by NIST is the NIST-F1, which operates on the fountain principle.::30 billionths of a second each year In 1967, the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the […], which operates on the fountain principle.::NIST-F1 In 1967, the cesium atom was formally recognized internationally as the time standard. In the SI unit definition of a second, it states that there are 9192631770 oscillations of the atom's resonant frequency, rather than in terms of Earth motions, as the old standard was. As of 2002, the latest standard is accurate up to 30 billionths of a second each year. The latest clock built by NIST is the NIST-F1, which operates on the […].::fountain principle Most clocks and watches today keep time by applying […] to a quartz crystal, a system developed in the 1930s.::electrical energy Most clocks and watches today keep time by applying electrical energy to […], a system developed in the 1930s.::a quartz crystal Most clocks and watches today keep time by applying electrical energy to a quartz crystal, a system developed in […].::the 1930s […] work well because they have no gears or escapements that disturb their regular frequency. However, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size, shape and temperature. This means no two crystals have exactly the same frequency numbers, although quartz clocks still dominate the market due to their performance for their price. However, in terms of overall performance, atomic clocks are considered superior.::Quartz crystal clocks Quartz crystal clocks work well because they have no […] that disturb their regular frequency. However, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size, shape and temperature. This means no two crystals have exactly the same frequency numbers, although quartz clocks still dominate the market due to their performance for their price. However, in terms of overall performance, atomic clocks are considered superior.::gears or escapements Quartz crystal clocks work well because they have no gears or escapements that disturb their regular frequency. However, they still relied on a mechanical vibration whose frequency depended critically on […]. This means no two crystals have exactly the same frequency numbers, although quartz clocks still dominate the market due to their performance for their price. However, in terms of overall performance, atomic clocks are considered superior.::the crystal's size, shape and temperature Quartz crystal clocks work well because they have no gears or escapements that disturb their regular frequency. However, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size, shape and temperature. This means no two crystals have exactly the same […], although quartz clocks still dominate the market due to their performance for their price. However, in terms of overall performance, atomic clocks are considered superior.::frequency numbers Quartz crystal clocks work well because they have no gears or escapements that disturb their regular frequency. However, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size, shape and temperature. This means no two crystals have exactly the same frequency numbers, although quartz clocks still dominate the market due to their performance for their price. However, in terms of overall performance, […] are considered superior.::atomic clocks […] use a coiled mainspring for power, where it drives gears that cause a hairspring to oscillate, which then rocks a lever to and fro, which then drives other gears to move the hands.::Mechanical watches Mechanical watches use a […] for power, where it drives gears that cause a hairspring to oscillate, which then rocks a lever to and fro, which then drives other gears to move the hands.::coiled mainspring Mechanical watches use a coiled mainspring for power, where it drives gears that cause a […] to oscillate, which then rocks a lever to and fro, which then drives other gears to move the hands.::hairspring Mechanical watches use a coiled mainspring for power, where it drives gears that cause a hairspring to oscillate, which then rocks a […] to and fro, which then drives other gears to move the hands.::lever Mechanical watches use a coiled mainspring for power, where it drives gears that cause a hairspring to oscillate, which then rocks a lever to and fro, which then drives other gears to move […].::the hands The world's most accurate timekeepers are::Atomic clocks […] are accurate to 1 second in 400,000 years, or within a millionth of a second per year. Other types of atomic clocks which do not use cesium have also been developed, such as a hydrogen clock known for being very stable, or clocks based on microwave absorption in rubidium, which are more compact, lower in cost, and less power demanding.::Ground based atomic clocks Ground based atomic clocks are accurate to […]. Other types of atomic clocks which do not use cesium have also been developed, such as a hydrogen clock known for being very stable, or clocks based on microwave absorption in rubidium, which are more compact, lower in cost, and less power demanding.::1 second in 400,000 years, or within a millionth of a second per year Ground based atomic clocks are accurate to 1 second in 400,000 years, or within a millionth of a second per year. Other types of atomic clocks which do not use […] have also been developed, such as a hydrogen clock known for being very stable, or clocks based on microwave absorption in rubidium, which are more compact, lower in cost, and less power demanding.::cesium Ground based atomic clocks are accurate to 1 second in 400,000 years, or within a millionth of a second per year. Other types of atomic clocks which do not use cesium have also been developed, such as a […] known for being very stable, or clocks based on microwave absorption in rubidium, which are more compact, lower in cost, and less power demanding.::hydrogen clock Ground based atomic clocks are accurate to 1 second in 400,000 years, or within a millionth of a second per year. Other types of atomic clocks which do not use cesium have also been developed, such as a hydrogen clock known for being very stable, or clocks based on […], which are more compact, lower in cost, and less power demanding.::microwave absorption in rubidium Ground based atomic clocks are accurate to 1 second in 400,000 years, or within a millionth of a second per year. Other types of atomic clocks which do not use cesium have also been developed, such as a hydrogen clock known for being very stable, or clocks based on microwave absorption in rubidium, which are […].::more compact, lower in cost, and less power demanding In […], the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::1830 In 1830, […] established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::the US Navy In 1830, the US Navy established a depot which later became […]. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::the US Naval Observatory, abbreviated USNO In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for […] and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::marine chronometers and other navigation instruments In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to […]. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::"rate" the chronometers to assure their accuracy for their use in celestial navigation In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required […]. In December of 1854, the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::regular astronomical observation In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In […], the Secretary of the Navy finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::December of 1854 In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the […] finally designated this institution as the United States Naval Observatory and Hydrographic Office. To this day, timekeeping is still one of the functions of it.::Secretary of the Navy In 1830, the US Navy established a depot which later became the US Naval Observatory, abbreviated USNO. The initial responsibility of this organization was to serve as a storage site for marine chronometers and other navigation instruments and to "rate" the chronometers to assure their accuracy for their use in celestial navigation. This later task, if done well, required regular astronomical observation. In December of 1854, the Secretary of the Navy finally designated this institution as […]. To this day, timekeeping is still one of the functions of it.::the United States Naval Observatory and Hydrographic Office In […], a railway standard time for England, Scotland, and Wales replaced several "local time" systems.::the 1840s In the 1840s, a […] for England, Scotland, and Wales replaced several "local time" systems.::railway standard time In the 1840s, a railway standard time for […] replaced several "local time" systems.::England, Scotland, and Wales In the 1840s, a railway standard time for England, Scotland, and Wales replaced […].::several "local time" systems